The present invention relates in general to a method for depositing a layer of material, such as a metal, onto a substrate surface, such as a semiconductor wafer. The method of this invention is particularly advantageous for enhancing metal deposition over and within complex, minuscule surface structures such as those encountered in the Ultra Large Scale Integration (ULSI) and "beyond" technologies of semiconductor device fabrication.
Microelectronic structures frequently use thin, conductive metal layers as device interconnects. The device interconnects provide electrical connection between devices ultimately formed within and upon the substrate. Among the metals commonly used in the formation of such interconnects are tungsten and titanium. Ideally, in the formation of such interconnects, a metal layer is deposited uniformly across the topography of the substrate surface and uniformly extends within small grooves, channels and openings provided within the surface. Such grooves, channels and openings generally define the ultimate location of device interconnects. Uniformly filling such grooves, channels and openings becomes increasingly difficult as devices, and their interconnects, become increasingly more miniature.
A prior art method for providing a conductive metal layer over a semiconductor substrate is sputter deposition. A sputter deposition apparatus, and a method for using such apparatus to deposit metal onto a substrate, is described with reference to FIGS. 1 and 2. Apparatus 10 comprises an outer wall 12 surrounding a reaction chamber 14. Outer wall 12 comprises an interior surface 11 and an exterior surface 13. A plurality of ports 16 extend through outer wall 12 and thereby enable gases to be flowed into and out of reaction chamber 14. In the shown embodiment, two such ports 16 are provided, one of which is in fluid connection to gas sources 18 and 20 and the other of which is in fluid connection to a pump 22. In operation, gases flow from sources 18 and 20 through reaction chamber 14 and toward pump 22. A series of valves (not shown) regulates the rate of flow of such gases. The gases flowed are generally noble gases, such as argon, helium or neon, or mixtures thereof.
The shown apparatus comprises two gas sources, and is thereby configured for flowing a gas mixture into reaction chamber 14. This is typical of apparatuses utilized for conducting sputter deposition.
Within reaction chamber 14 there is provided a target 24, a collimator 26 and a substrate 28. Target 24 comprises an erodible surface 30. Erodible surface 30 contains atoms which are ultimately to be sputter deposited onto an exposed surface 29 of substrate 28. A power source 32 is provided in electrical connection with target 24. Power source 32 may be any of a number of sources known to persons of ordinary skill in the art, including: Direct Current (DC) sources, Alternating Current (AC) sources, and Radio Frequency (RF) sources. Also, a matching network (not shown) may be placed between power source 32 and target 24 for impedance matching the source to the reaction chamber. Generally, as shown, outer wall 12 is grounded, and most generally both the interior surface 11 and the exterior surface 13 of outer wall 12 are grounded.
Substrate 28 is typically in electrical connection to an electrical component 34 exterior to reaction chamber 14. Component 34 may simply be a ground, or it may be some other device generally known to persons of ordinary skill in the art, including, an RF generator, an AC source, or a DC source. Also, a matching network (not shown) may be placed between component 34 and substrate 28 for matching the impedance of component 34 with reaction chamber 14.
In operation, a noble gas is flowed into chamber 14, and a target power is established with power source 32 to generate a plasma from the gas within chamber 14. As mentioned above, most commonly the gas used for generation of such plasma will be argon, but mixtures of argon with helium and/or neon are also occasionally used.
The power source 32 and electrical component 34 together maintain an electric field within chamber 14 which directs plasma ions toward erodible surface 30 of target 24. As the ions collide with erodible surface 30, target atoms are sputtered therefrom. The sputtered target atoms eject from target 24 over a wide angular spread of trajectories. Some of these target atoms reach surface 29 of substrate 28, become deposited onto the surface, and accordingly form a layer of target atoms across the surface 29.
Collimator 26, if present, selectively filters the sputtered atoms having particular trajectories to create a stream of sputtered atoms which are substantially parallel with one another. By such selective filtering of the sputtered atoms, collimator 26 helps to ensure that a uniform layer of target atoms will be deposited across the flat surfaces of substrate 28. Particularly, collimator 26 can help to ensure that sputtered material goes into, and fills, small grooves, channels and openings in the substrate, rather than simply creating a skin across such structures. Collimator 26 may be charged, grounded, or electrically floated within reaction chamber 14.
Even with a collimator 26, it is difficult to achieve highly uniform coverage of target atoms across a substrate surface. One of the reasons for this difficulty may be that target atoms collide with particles in the reactor--such as plasma components, background (neutral) gas, or each other--after passing the collimator and thereby acquire a trajectory non-parallel to the trajectories of other target atoms. If sputtered target atoms approach surface 29 from multiple angular trajectories, the target atoms are more likely to deposit on the outer edges of small grooves, channels and openings, rather than within such structures. Once target atoms adhere to the outer edges of such structures, the openings into the structures are narrowed. It therefore becomes increasingly difficult to fill the structures and increasingly more likely that the structures will simply be covered, rather than filled. This problem becomes exacerbated with smaller devices, wherein the openings are exceedingly small to start with, and wherein relatively minor differences in the trajectories of sputtered target atoms can lead to gross defects in the uniformity of coverage across surface topologies of surface 29.
A method for reducing the probability of the above-described undesired collisions of target atoms with plasma components would be to develop a plasma which could be sustained at very low pressures, for a given power, and to then reduce the pressure within reaction chamber 14 to the threshold pressure of the plasma, i.e, to the pressure below which the plasma will no longer exist.
Presently, such minimization of threshold plasma pressure is not utilized with sputtering processes. Instead, the state of the art is to perform a calculation, such as the throw distance calculation discussed in U.S. Pat. No. 5,114,556, which balances the need for rapid deposition with a desire to decrease pressure. However, with devices becoming increasingly miniaturized, it is becoming increasingly important to develop plasmas which can be sustained at very low pressures, hereafter referred to as "low-pressure plasmas". A complication with developing such low pressure plasmas is that the ideal composition of the plasma will vary depending on the configuration of the reaction chamber 14. A general method for determining such an ideal composition for any reaction chamber configuration, and for ascertaining the threshold plasma pressure of such an ideal composition, is addressed by the present invention.